storms and may range from a fraction of an inch (which
amount of water, and consequently rapidly decrease the
may not cause appreciable flooding) to an amount
area of inundation at Yucca Lake, Nevada Test Site,
greater than the total rainfall of the preceding year
Nevada (Doty and Rush 1985). At Rogers Playa, the
(which can very quickly flood a playa) (Stone 1956). In
bulk of a 2.54-cm (1-in.) rainfall in 1983 is reported to
an examination of 45 California playas over three con-
have drained into a newly formed desiccation crack with-
secutive years, Kubly (1982) reported that during 1978,
in 24 hours (Blodgett and Williams 1990). Interestingly,
65% of playas ponded water, but 45% and 30% ponded
the shifting of water back and forth due to wind may
water in 1979 and 1980, respectively (see also Table 1,
smooth out irregularities in the surface. This smooth-
pg. 14). The inundation period is influenced by many
ing factor may have application in determining the extent
variables, including the geometric, geomorphic, soil, and
of previously ponded water. Data on evaporation rates
vegetation characteristics, as well as evaporation rates
could provide information useful to OHW delineation,
(as influenced by climatic conditions, water salinity, and
but few are available. Stone (1956) estimates the aver-
geometry of the water body itself). Some playa surfaces
age annual evaporation rates in southern California
are impervious to infiltration of surface water while oth-
deserts at 203.2228.6 cm (8090 in.) per year. In apply-
ers have enormous capacity for absorbing and trans-
ing this figure to a 2.44-m (8-ft) flooding (normally, pla-
porting moisture (Neal 1965). In yet other instances,
yas flood to depths of only a few inches) of Silver Dry
groundwater may be the predominant source of surface
Lake, he reported the calculated figure to be a slight
water to a particular playa; in these instances, the water
underestimate. Some examples of evaporation times and
level may be stable or at least less variable than playas
rates are given in Table 1. Interestingly, salt crusts can
whose surface water budget is largely dependent on
reduce water evaporation rates to 2% of non-salt-
precipitation (these geomorphic characteristics are dis-
crusted surfaces (Chen 1992) and may be associated
cussed below).
with groundwater levels at or nearer the surface than in
The spatial extent of ponding varies as well; Kubly
the absence of the crust.
(1982) reports that on any given playa the extent of water
Data pertaining to groundwater level of soft playas
coverage ranged from small, scattered ponded regions
are similarly scarce. Many soft playas (see below) tend
to more than 90% of the surface area. One serious con-
to have dry surfaces during the summer and wet during
founding influence for delineations based either on the
the winter (e.g., Stone 1956). On playas in Pilot Valley,
extent of ponded water or remnants ("indicators") on
Utah, the water tables vary from 3 to 5 cm (1.18 to 1.96
dry surfaces is that of wind-induced water movement.
in.) from the surface during the wet season to about 30
Stone (1956 and references contained therein) describes
cm (11.81 in.) during the dry season, though the soil
instances in which water may shift considerable
remains moist all year (Malek et al. 1990). When ground-
distances, or be driven toward one end of a playa dur-
water is at the lower level, a salt crust forms.
ing a windstorm. Malek et al. (1990) report that the area
ponded with water in the playa in Pilot Valley, Utah, may
INDICATORS OF ANTECEDENT
move several miles in response to changes in wind di-
HYDROLOGIC CONDITIONS
rection. Sheets of water have been observed to breach
minor drainage divides on playa surfaces during peri-
Once standing water has evaporated from or perco-
ods of high wind (Lines 1979), thus resulting in chang-
lated into the substratum at a particular site, a range of
es in the areal extent of coverage until the next flooding
indicators may remain or develop that can be used to
or drying cycle. Motts (1972) commented on the high rate
provide evidence of previous ponding. Characteristics
of water movement on playas (Rogers and Rosamond Pla-
of playas that might provide information on antecedent
yas, California) in relation to wind velocity; although he
hydrology are discussed here from the perspectives of
reported rates of up to 1.82 m (6 ft) per minute in response
surface morphology, soils, and vegetation.
to a 42-mph wind, he did not report on the areas occupied
Surface morphology
by standing water. Also on Rogers Playa, Dinehart and
McPherson (1998) report wind-induced changes in water
The surface morphology of most playas is related to
depth of more than 0.3048 m (1 ft).
several factors, the most important being the ratio be-
Another important caveat is that drastic changes in
tween surface-water flooding and capillary discharge
the areas inundated can occur over relatively short pe-
from groundwater (Motts 1970). There have been sev-
riods because of anthropogenic and natural changes in
eral attempts to classify playas and playa surfaces on
groundwater level causing land subsidence and the for-
the basis of this ratio because of the characteristic
mation of giant desiccation cracks (e.g., Blodgett and
appearance of playas at each extreme (e.g., Stone 1956,
Williams 1990). Recently opened giant desiccation
Neal 1965, and Stevens 1988). However, because sur-
cracks have been documented to drain a substantial
face types may vary with time and intergrade, the value
5
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